1. Field of the Invention
This invention relates to a light emitting device made of nitride compound semiconductor of III-V group (in general, expressed in a formula of InXAlYGa1-X-YN, wherein 0<X, 0≦Y, X+Y≦1). And especially it relates to a light emitting device including an active layer depositing at least two kind of well layers emitting different colors of light and mixing colors thereby emitting light of another color such as white with a desired color rendering property.
2. Description of Related Art
Since light emitting devices which emit light of red, green, and blue color (so-called RGB) with quite strong intensity have been developed so far, RGB light emitting devices of high luminous intensity type are now manufactured and available. It is noted that the color of light emitted by the nitride compound semiconductor device can be varied from ultraviolet region through red region, by adjusting the composition ratio thereof.
Meanwhile, because of excellent characteristics of the light emitting devices such as high luminous intensity, downsizing, and high credibility, applications thereof have been rapidly expanding in the technical fields, for example, for light sources for indicators mounted on automobiles, light sources for backlights of liquid crystal displays, and other luminaires.
In those applications, especially since white light emitting devices are easiest and most comfortable to human eyes, such white light emitting devices are highly desired. There are basically two approaches to obtain such white light. Three of RGB or two of blue and yellow (that is complementary color of blue) color light emitting diode chips are mounted on a same stem to mix them to produce the white light. Alternatively fluorescer emitting fluorescent yellow light when absorbing blue light are applied on the blue color emitting diode chips to produce white light.
However those approaches to achieve white light have some problems. When mounting a plurality of light emitting diode chips on the same stem to produce white light, those chips have to be disposed as closely as possible each other in order to improve a color mixing property. (The color mixing property is referred herein denote the extent to which the light from the device can be evenly seen as single white light.) But the finite sizes of the chips limit the improvement. Further when utilizing chips of different semiconductor based materials, they have different forward voltages and distinct characteristics dependent upon the temperature.
Also when utilizing fluorescer to produce the white light, a step for applying fluorescer on the chip is required, which is a complicated step. Further when combining the blue light emitting chip and fluorescer emitting fluorescent yellow light by absorbing the blue light to obtain white light, the luminous efficiency is theoretically reduced, in comparison with the combination of a plurality of different color chips.
Therefore those conventional approaches are not yet satisfactory to replace the current light sources with the semiconductor light emitting devices, and it is desired to provide the light emitting devices with higher luminous efficiency and higher luminous intensity that are capable of emitting white light. This invention is directed to providing such a light emitting device which can emit white light by itself.